I found out how to do this when learning how websites get the real client IP behind a tranparent proxy. So first I will explain to you how this works.
So, when a transparent proxy sends a request, it sends a HTTP header by the name of "X-Forwarded-For" which contains the real IP of the user of the proxy. So this is what we will spoof our IP to anything we want.
1. Get FireFox
2. Get this plugin: https://addons.mozilla.org/en-US/firefox...y-headers/
3. Once installed you will see an icon like this one:
It will be to the right of the address bar.
4. Click it, and then click "Open ModifyHeaders"
A screen will come up that looks like this:
5. In there you will fill out the section that looks like this: